fix: make backend listen on 0.0.0.0 instead of localhost

This allows Traefik and other containers on the docker network to reach the backend API.

backend/src/index.js

@@ -394,7 +394,7 @@ app.get('/api/today/:programId', async (req, res) => {
   }
 });
 
-app.listen(PORT, () => {
+app.listen(PORT, '0.0.0.0', () => {
   console.log(`Gravl API running on port ${PORT}`);
 });